Oil Temperature Advice

[JTO]

Hello All,

I have been reading the posts on this site for months. I finally became a member so I can pick the various brains and fill in the many blanks I have in my 911 knowledge basket.

The first question I have is about oil temperture. I recently had my engine and tranny rebuild (by Motor Meister-don't hate me. I'll go into great detail about the whole thing another time) and this summer saw oil temperatures on the order of 240 F. When I used an IR temperature gun to measure oil line and filter temperatures, I found them to be about 20 degrees cooler than the indicated oil temperature. I know about the emmisivity differences of the diffent materials, thats why I measured the temp. of a few different surfaces in contact with the oil. So, in summary, gauge says lets say 215 F and the IR gun says 190-197 depending on the oil line or oil filter. I have only the engine mounted oil cooler.

Do gauges ever read high? Which temperature is more believable and pertinent? The IR gun was calibrated and is of known accuracy.

Thanks for the input!

JTO

[tmctguer]

i was seeing very high oil temps at the track (220 - 230), so i put on a front fender well mounted cooler on my '73 911E. that reduced my temps to an acceptable level (190 - 200), so that's how i solved my problem.

sorry i can't help with the accuracy question.

[sithot]

If you don't have an oil cooler, you need one. 240 and more is a bit on the hot side. You may consider having your gauge and or sending unit calibrated/checked.

I have an IR gun to but only use it to check exhaust temperatures when I'm tuning a carburetor (on vintage BMW bikes). Works like a charm and you can see immediately whether the adjustments worked. Too fat-cold, too lean-hot. I've been able to get readings within 1-2 degrees side to side with this wonderful tool.
The fact that the fluid is not actually being "seen" by the IR gun would mean the temps would be lower.

North Hollywood speedometer or Palo Alto Speedometer might be of some assistance. They also do wonderful gauge restorations for antique motorcycles.

good luck.

[Early_S_Man]

JTO,

The engine oil temp sensor is located in the middle of an oil gallery where you would expect a temp gradient out to the walls of the gallery, and further temp drop outside to the surface of the engine block. Likewise with the hot oil in the tank ... surface temp readings on the tank would be expected to be lower than the oil flowing to the pressure pump!

I have done a temperature profile with an early numerical gauge and 'new old stock' VDO sensor in a pan of stale cooking oil on a hot plate, compared with a calibrated Fluke 52 Thermometer and stainless Type 'K' thermocouple probe ... from 75°F to 320°F. At none of the gauge marked lines was the Fluke reading off more than 4°F, so I consider that to be indicative of the factory gauges accuracy potential with new wiring and operating from a precision HP 6434B DC Power Supply. I made test runs with the supply Voltage set at 13.80 Volts and 14.20 Volts and found no difference in gauge readings!

I believe that old factory wiring and corroded connections could cause degraded accuracy, but cleaning the 1/4" Faston connectors at the gauge and sensor, as well as the engine harness 14-pin connector should restore gauge accuracy.

[island911]

calibrate your gauge and sending unit .

Maybe do this yourself. (?) hmmmm, what to calibrate against. . .something constant. . . there is a rumor that water boils right at 100°C, 212°F,. . . and due to latent heat properties, it stays there for a while.

[ChrisBennet]

The oil in the tank has already gone through the oil cooler(s) so it should be quite a bit cooler than the guage would read.
-Chris

[JTO]

Thanks Everyone for your advice. I think I will use the boiling water method to check the accuracy of the gauge and sender.